1. Field of the Invention
The invention relates generally to a malfunction determining device for a drive circuit, a drive unit including such malfunction determining device, and a method for determining whether a malfunction is present in the drive circuit. In particular, the invention relates to a malfunction determining device that determines whether a malfunction is present in a drive circuit that drives an electric device, which generates counter-electromotive force while running, using a plurality of switching elements, a drive unit including such malfunction determining device, and a method for determining whether a malfunction is present in the drive circuit.
2. Description of the Related Art Japanese Patent Application Publication No. JP-A-2000-134947 describes a malfunction determining device, which blocks gates of switching elements when detecting overcurrent supplied to the switching elements included in an inverter device. In such a device, an electric current sensor (e.g. a hole CT) is provided in series with a capacitor arranged between a positive bus bar and a negative bus bar that are connected to the switching elements of the inverter device, the hole CT detects overcurrent supplied to the switching elements, and the gates of the switching elements are blocked when the hole CT detects the overcurrent.
With the malfunction determining device for a drive circuit described above, however, even when the gates of the switching elements are blocked, if a failure is present in at least one of the switching elements and the switching element cannot be turned OFF (hereinafter, this failure will be referred to as an “ON-failure”), a supply of overcurrent is not detected. This may cause problems, for example, the inverter is damaged due to overheat. The inverter is used to drive a motor and an electric power generator. Many types of motors and electric power generators generate counter-electromotive force while running. Accordingly, if an ON failure occurs in a switching element of the inverter while the motor or the electric power generator, which generates such counter-electromotive force, is running, even if the gates of the switching elements in the inverter are blocked, a closed circuit is formed by the switching element, where the ON-failure is present, and a wound-coil of the motor or the electric power generator. As a result, electric current flows due to the counter-electromotive force generated while the motor or the electric power generating is running. Due to such electric current, although the gates are blocked, the switching elements overheat, resulting in possible damage to the inverter or motor.